In young coals from cellulosic plant material, the acids formed, such as humic, combined with alkalies from the ground water to form active compounds from which the alkalies are volatilized on burning. When the coal is thermally decomposed during combustion, these active alkalies first become rather large solid particles of sodium oxide (Na.sub.2 O) which remain firmly imbedded in the carbon residue of the thermally decomposed organic acids. The result of the first stage of combustion on such mineral constituents of the coal is expressed by the following equation: ##STR1##
The sodium oxide so produced is a relatively refractory compound with a high boiling point of approximately 2300.degree. F. Sodium oxide is, therefore, difficult to volatilize from the carbon matrix formed in this initial, low-temperature stage of coal combustion. By contrast, the sodium metal vapor next formed by "reduction" of sodium oxide with carbon in the second stage of coal combustion has a boiling point of only 1620.degree. F., which is 680.degree. F. lower than that of parent sodium oxide. The fate of the alkali ash constituent in this second, higher temperature stage of combustion is expressed by the following equation: (2) Na.sub.2 O (massive)+C.fwdarw.Na.sub.2 (vapor)+CO (gas). Note that "reduction" is a chemical reaction in which oxygen is stripped from a compound by a reducing agent. In the above case, sodium oxide has been reduced to sodium metal vapor and the carbon has been oxidized to carbon monoxide.
At normal high furnace temperatures, the highly volatile sodium metal vapor therefore "spews out" forcefully from the burning fuel particle into the surrounding air envelope as soon as it is formed. The sodium metal vapor is highly reactive with air and thus burns quickly to form sodium oxide fume. This result is expressed by the following equation: (3) Na.sub.2 (vapor)+1/2O.sub.2 .fwdarw.Na.sub.2 O (fume). This fume is a very fine, sub-micron sized dust which is the normal state of division of the solid ash particles from burning a metal vapor. The sodium oxide fume from equation 3 is radically different from the initial massive form of sodium oxide produced by thermal decomposition of sodium humate represented by equation 1. For instance, the fume exists as a chemically reactive aerosol in the flue gas stream. Its particles have a very high specific surface area and readily react with the surface of the initially powdery silica-rich ash deposits on boiler tubes to form sticky ash-bonding alkali silicates. The larger crystalline sodium oxide particles from equation 1, by contrast, are not dispersed in the flue gas stream since they are anchored in the fuel particle by the carbon matrix until the key reduction reaction 2 occurs.
When the third stage of combustion is reached and a sodium oxide aerosol in flue gas is produced, it will sweep over the initially powdery ash deposits on the tubes. This deposition of the sodium compound from the aerosol will form a surface coating of sticky, low-melting ash-bonding alkali silicates on each silica particle. This result is represented by the following generic equation: (4) Na.sub.2 O (fume)+SiO.sub.2 (in ash).fwdarw.Na.sub.2 SiO.sub.3 (silicate glass).
Some of the young coals, such as lignite and subbituminous, found in at least the Western United States, have a sufficiently high alkali (sodium) content to give the rapid ash-fouling problem. When these coals are burned in utility boilers, the resulting ash sticking to heat transfer surfaces quickly builds up in thick insulating layers and is very expensive to remove. Therefore, these problem coals cause prohibitive ash fouling and repeated unscheduled shutdowns for manual removal. Attempts to overcome this problem have been expedient, often mechanical, and of limited effectiveness. Worse, the symptoms, rather than the "disease", have been attacked.
First, the furnace design parameters such as volumetric heat release rate and furnace (flue gas) outlet temperatures have been descreased in an effort to remedy this fouling. Second, elaborate systems for steam and water sootblowing have also been applied. These approaches have been in the nature of a quick fix, temporarily effective, but they have constituted no fundamental solution.
The relationship between the presence of active alkalies in coal which are soluble in dilute acid and are readily volatilized upon combustion, and the resulting fouling of furnace surfaces has been well established by fundamental research. Therefore, a third solution to the fouling problem has been sought through leaching the soluble alkali compounds from the finely ground coal using dilute aqueous acids. Again, this solution has proved impractical because of its expense and the fact that excessive water remained in the coal.
The fourth solution suggested was the use of an "alkali-getter". Specifically, finely-divided silica or alumina was proposed as an additive to combine with the volatilized alkali and form a high-melting end product in ash deposited on the tube surfaces. However, there has been no development of this solution to bring it up to a commercial level.
A fifth approach has been hinted at by physically absorbing the molten ingredient(s) from an ash deposit which cause the ash particles to adhere to one another and to the tube surface, by use of a porous siliceous type additive like diatomaceous earth.
The searches for a practical solution to the ash fouling problem have even included coal blending to eliminate the problem. This has good troublesome and expensive. The foregoing approaches are examples of measures which have been and are now being studied to alleviate the serious ash-fouling problems with high sodium coals.
To summarize the ash-fouling mechanism, the basic function of the key chemical reduction reaction 2, and sequential reaction 3, is the creation of an aerosol of highly reactive sodium oxide fume in the flue gas which generates the low-melting sticky "glue" in deposited powdery ash (be reaction 4) which bonds it to the tubes and the ash particles to one another. Metallic sodium vapor, the precursor of this "glue", can be formed only if all three of the following conditions are maintained during the coal combustion process: (a) a highly reducing (oxygen deficient) atmosphere at the coal particle-to-gas interface, (b) a high enough temperature to effect reaction 2, and (c) a long enough reaction time. Under equilibrium conditions this reaction is most likely to occur above 1926.degree. F., the threshold temperature. Sodium metal vapor production accelerates rapidly as the reaction temperature increases above this value when carbon is present. Changes in the coal-firing method which both increase the initial contact of fuel with free oxygen (to decrease the intensity of and exposure time to reducing conditions) and also control the burning temperature to below a predetermined maximum value will decrease the rate and amount of metallic sodium vapor formed and the resulting ash fouling.